In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
The installation of anchor bolts to reinforce excavations is usually carried out in two distinct steps. Usually, a bore is drilled and the drill steel and bit extracted before the bolt is inserted into the bore and tightened or grouted. Single pass anchor bolting involves carrying out these two steps simultaneously, with the task of removing the drill steel to insert the bolt being eliminated. The advantages of single pass bolting include minimizing the time required for bolt installation, improving safety for drilling equipment operators, when comparing with manual or semi manual bolting, and enhancing prospects for full automation of the process. A further advantage is improved quality and precision of anchor bolt installation, when comparing with manual or semi manual bolting. The diameter of the bore is critical for anchor bolt performance in the case of friction, e.g. Split set bolts. Still a further advantage with single pass bolting is that the bore cannot collapse when retracting the drill bit since the bolt is already in the bore. This leads to much better efficiency as the bolt is always installed; i.e., there will be no lost holes.
Prior attempts at single pass bolting have generally been targeted at innovative anchor bolts, which also act as the drill steel, having a drill bit provided about an end thereof. Such apparatus are used via a rotational drilling method or a rotary/percussive drilling method and are generally unsuitable for hard ground conditions. Existing hard ground percussive anchor bolts that do not reuse the drill bit suffer from cost problems. A wide variety of roof bolts exist and one particular form is tubular (e.g. split-sets, Swellex, etc. . . . ), having a central bore formed lengthwise through the bolt. Drill bits adapted to be extracted through a casing have been complex and accordingly expensive.
A drill bit has been proposed in Swedish Patent Application No. 0400597-1 that has a reamer part that incorporates rock machining elements which are disposed non-symmetrically about the drill bit axis. This construction allows a bore diameter to be formed that is larger than the diameter of the drill bit, thereby allowing removal of the bit through a casing. However, the penetration profile of the drill bit has been found to be non-uniform thereby causing stress on the bit which can reduce the bit's efficiency and service life.
Cost competitiveness of drilling speed versus bit cost are complicated in prior single pass anchor bolts due to the use of specialized anchor bolts and the exclusive use of either complex retractable bits. It nevertheless remains the case, that the installation advantages of a self-drilling roof bolt outweigh those of the non-self-drilling type.